Superconducting cable having a flexible former

ABSTRACT

In order to provide a flexible oxide superconducting cable which is reduced in AC loss, tape-shaped superconducting wires covered with a stabilizing metal are wound on a flexible former. The superconducting wires are preferably laid on the former at a bending strain of not more than 0.2%. In laying on the former, a number of tape-shaped superconducting wires are laid on a core member in a side-by-side manner, to form a first layer. A prescribed number of tape-shaped superconducting wires are laid on top of the first layer in a side-by-side manner, to form a second layer. The former may be made of a metal, plastic, reinforced plastic, polymer, or a composite and provides flexibility to the superconducting wires and the cable formed therewith.

CROSS REFERENCE TO RELATED APPLICATION

This application a division of patent application Ser. No. 10/226,177filed Aug. 22, 2002, now U.S. Pat. No. 6,867,375 which is a division ofSer. No. 09/392,278, filed Sep. 9, 1999 now U.S. Pat. No. 6,596,945which claims benefit of 60/099,909 Sep. 11, 1998 relied on andincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconducting cable employing aflexible oxide superconductor, and more particularly, it relates toforming a superconducting cable.

2. Description of the Background Art

Superconducting materials are those where the electric resistanceapproaches zero (1 uv/cm) below a critical temperature, its valuedepending on the material. Superconductivity is defined within acritical surface, i.e. a graph or figure with its axes beingtemperature, electrical current and magnetic field. Thus, for a givenworking temperature there is a defined curve of critical current whichis a function of the magnetic field generated and/or applied to thesuperconductor.

The best known superconductor materials are NbTi and Nb₃Sn, howevertheir working temperature is only 4.2K, the boiling temperature ofliquid helium. This is the main limitation to large scale application ofthese superconducting materials. Such superconductors are therefore usedalmost exclusively for winding of magnets. Manufactured from wires (NbTiand Nb₃Sn) or tapes (Nb₃Sn) with high critical current densities (3500A/mm² 5 Tesla for NbTi), such winding of compact magnets provide theproduction of high fields (up to 18 Tesla) in large volumes.

These superconductor magnets are used for the formation of medicalimages by nuclear magnetic resonance (MRI) and for materials analysis bythe same principle (NMR), the magnets for ore separation and researchmagnets for high fields, such as those used in large particleaccelerators (SSC, HERA, KEK, etc.).

Oxide superconductors of higher critical temperatures were discovered in1986. These are intermetallic compounds involving metal oxides and rareearths, with perovskite (mica) crystal structure. Their criticaltemperatures vary from 30K to approaching room temperature and theircritical fields are above 60 Tesla. Therefore these materials areconsidered promising and may replace Nb₃Sn and NbTi in the manufactureof magnets and find other applications not feasible with liquid helium,such as transmission of electricity. Such materials have not previouslybeen available as wires, cables, films, tapes or sheets.

An oxide superconductor which enters the superconducting state at thetemperature of liquid nitrogen would be advantageous for application ina superconducting cable having a cooling medium of liquid nitrogen. Withsuch an application, it would be possible to simultaneously attainsimplification of the thermal protection system and reduction of thecooling cost in relation to a superconducting cable which requiresliquid helium.

A superconducting cable must be capable of transmitting high currentwith low energy loss in a compact conductor. Power transmission isgenerally made through an alternating current, and a superconductoremployed under an alternating current would inevitably be accompanied byenergy loss, generically called AC loss. AC losses such as hysteresisloss, coupling loss, or eddy current loss depends on the criticalcurrent density of the superconductor, size of filaments, the structureof the conductor, and the like.

Various types of superconducting cables have been experimentallyproduced using metallic superconductors to study the structures forreducing AC loss, such as a superconductor which comprises a normalconductor and composite multifilamentary superconductors which arespirally wound along the outer periphery of the normal conductor. Theconductor is formed by clockwisely and counterclockwise wound layers ofcomposite multifilamentary superconductors, which are alternatelysuperimposed with each other. The directions for winding the conductorsare varied every layer for reducing magnetic fields generated in theconductors, thereby reducing impedance and increasing current carryingcapacity thereof. This conductor has a high-resistance or insulatinglayer between the layers.

When a cable conductor is formed using an oxide superconductor, thetechnique employed in a metal superconductor cannot be used. An oxidesuperconductor, i.e., a ceramic superconductor, is fragile and weak inmechanical strain compared with a metal superconductor. For example, theprior art discloses a technique of spirally winding superconductorsaround a normal conductor so that the winding pitch is equal to thediameter of each superconductor. However, when a superconducting wirecomprising an oxide superconductor covered with a silver sheath is woundat such a short pitch, there is a high probability that the oxidesuperconductor will be broken, thereby interrupting the current. When anoxide superconducting wire is extremely bent, its critical current mayalso be greatly reduced.

The cable conductor must be flexible to some extent to facilitatehandling. It is also difficult to manufacture a flexible cable conductorfrom a hard, fragile oxide superconductor.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a superconducting cablehaving flexibility and exhibiting excellent superconductivity,particularly high critical current and high critical current density,having an oxide superconductor.

Another object of the present invention is to provide such asuperconducting cable which is reduced in AC loss.

According to the present invention a superconducting cable is providedemploying an oxide superconductor, which comprises a flexible coremember, and a plurality of tape-shaped oxide superconducting wires whichare wound on the core member, without an electric insulating layerbetween the superconducting wires or between the core member and thesuperconducting wires. In the inventive conductor, each of the oxidesuperconducting wires consists essentially of an oxide superconductorand a stabilizing metal covering the same. The plurality of tape-shapedsuperconducting wires laid on the core member form a plurality oflayers, each of which is formed by laying a plurality of tape-shapedsuperconducting wires in a side-by-side manner. The plurality of layersare successively stacked on the core member. This core member providesthe inventive superconducting cable with flexibility. Thesuperconducting cable according to the present invention maintains asuperconducting state at the temperature of liquid nitrogen.

The conductor according to the present invention further provides an ACconductor which is reduced in AC loss.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the multilayer structure of thepresent invention;

FIG. 2 is a sectional side view showing one embodiment of the presentinvention;

FIG. 3 is a sectional side view showing another embodiment of thepresent invention;

FIG. 4 is a depiction of the embossing pattern used in the presentinvention.

Referring to FIG. 1, superconductor cable 10 is shown having flexible,evacuated double walled, outer pipe 11, through which liquid nitrogen,12, flows to a chiller. Ground-potential superconductive shield material17 encircles dielectric layer 16, which in turn surrounds currentcarrying superconductive material 15. The flexible, porous-walled,former, 13, is encircled by superconducting material 15 and provides acentral, tube-like portion for transport of liquid nitrogen from thechiller. In one embodiment former 13 further has a surface that contactssuperconductive material 15.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a high temperature superconductor cablewhich may be used in the shielded or unshielded form of construction.There are many applications where both shielded and unshielded cablesserve useful purposes.

A modification of this embodiment is to insulate the cable withdielectrics over the high temperature superconductor tapes and thenprovide another high temperature superconductor layer over thedielectric. The entire cable is then either introduced into a cryostatof the type described above or a cryostat is constructed over the cable.This coaxial construction forces the magnetic field to stay between theinner and the outer layers of high temperature superconductor tapes.There is substantially no magnetic field outside the high temperaturesuperconductor tapes and therefore there is no eddy current in the outermetallic enclosures. With this construction very large amounts ofcurrent can be carried depending upon the number of tapes present in thecable. The limitation in this cable design is that the dielectricremains at the cryogenic temperature and a material which can withstandthe cryogenic temperature without any physical and mechanicaldegradation has to be used. The polymeric dielectric material of oneembodiment of the present invention has good physical and mechanicalproperties at liquid nitrogen and lower temperatures. It has highdielectric strength and high breakdown voltage. Advantageously the cableof the present invention includes the use of a flexible stainless steelcorrugated pipe, which is optionally covered with a wire braid or mesh.Preferably, the corrugated pipe is drilled with holes of a size andpattern to allow the liquid nitrogen to flow into the butt gaps of thehigh temperature superconductor tapes and flood the dielectric material.The high temperature superconductor tapes are laid in a special mannerto simulate two layer construction allowing maximum current to flowthrough the cable.

The dielectric material advantageously consists of semi-conductive tape,aluminized shield tape, and polymeric dielectric tapes. A typicalconstruction of a shielded cable is shown in FIG. 3. An unshielded cablecan be constructed by omitting the outer layers of high temperaturesuperconductor tapes. This cable construction is shown in FIG. 2. Thepresent invention includes both shielded and unshielded high temperaturesuperconductor cable. The design differs from other known cables in thecase of an unshielded cable where an extrusion of dielectric material isperformed over the thermal insulation cryostat. The prior art does notdisclose any method of construction for shielded high temperaturesuperconductor cable.

Referring to FIG. 1, superconductor cable 10 is shown having flexible,evacuated double walled, outer pipe 11, through which liquid nitrogen,12, flows to a chiller. Ground-potential superconductive shield material17 encircles dielectric and shield layer 16, which in turn surroundscurrent carrying superconductive material 15. The flexible,porous-walled inner pipe, 13, is encircled by superconducting material15 and provides a central, tube-like portion for transport of liquidnitrogen from the chiller. In one embodiment pipe 13 further has abraided surface that contacts superconductive material 15.

FIG. 2 illustrates an embodiment of an unshielded cable wherein former21 is surrounded by semiconductive bedding tape 22, upon which is laidsuperconductive tape 23. Another layer of semiconductive bedding tape 24surround superconductor tape 23. Shielding layer 25 encircles beddingtape 24 and dielectric layer 26 surrounds shielding layer 25. Dielectriclayer 26 is encircled by shield layer 27 which in turn is encircled bysemiconductive bedding layer 28. Bedding layer 28 is surrounded bybinder tape 29, which is encompassed by centering ring 30, in turnsurrounded by jacketed cryostat 31.

Referring to FIG. 3, which shows an embodiment of a shielded cable,jacketed cryostat 53 encompasses centering ring 52, which surroundsbinder tape 51, which in turn encircles semiconductive tape 50. Tape 50encircles superconductive tape 49, which surround semiconductive beddingtape 48, which encompass shielding layer 47. Dielectric 46 encirclesshield layer 45, which surrounds semiconductive tape 44. Superconductivetape 43 encircles semiconductive bedding tape 42, which surrounds former41.

The present invention relates to a cable employing an oxidesuperconductor comprising a flexible core member, a plurality oftape-shaped oxide superconducting wires laid on said core member withtension of not more than about 2 kgf/mm² and a bending strain of notmore than about 0.2% on the superconductor, wherein each tape-shapedsuperconducting wire consists essentially of an oxide superconductor anda stabilizing metal covering the same, said plurality of tape-shapedsuperconducting wires forming a plurality of layers each being formed bylaying said tape-shaped superconducting wires in a side-by-side manner,said plurality of layers being successively stacked on said core memberwithout an insulating layer between the plurality of layers and the coremember, said core member providing said superconducting cable withflexibility, said superconducting cable capable of maintaining asuperconducting state at the temperature of liquid nitrogen, said wireshaving substantially homogeneous superconducting phases along thelongitudinal direction of said wire, the c-axes of said superconductingphases being oriented substantially in parallel with the direction ofthickness of said wire, said superconducting wires being formed bygrains aligned in parallel extending along the longitudinal direction ofsaid wire, said grains being stacked along the direction of thickness ofsaid wire.

The superconducting cable advantageously has flexibility such that itssuperconductivity does not substantially deteriorate upon bending up toabout 50 times the diameter of the cable. It is also advantageous thatthe core member be selected from the group consisting essentially ofmetals, plastics, reinforced plastics, polymers, and composites. Oneembodiment of the superconducting cable provides a core member being apipe having a spiral groove surface, a web shaped surface, a mat shapedsurface, or a braid shaped surface on its exterior which forms a surfacefor the tape-shaped superconducting wires. The inventive superconductingcable does not have any insulating layer between the plurality of layersof the tape-shaped superconducting wires. Advantageously the tape-shapedwires are laid on said core member with the tape-shaped plurality oflayers being laid on the surfaces formed by said immediately prior layerof tape-shaped wires. In another embodiment the wires are twisted withinsaid tape-shaped stabilizing metal covering. Advantageously in thesuperconducting cable said tape-shaped wires are laid at a lay angle ofup to about 90 degrees, advantageously from about 10 to about 60degrees, and preferably from about 20 to about 40 degrees. Oneembodiment of the present invention includes a superconducting cablehaving at least two distinct groups of tape-shaped wire layers.Advantageously the lay angle of each successive layer of tape-shapedwires alternate in lay direction or pitch; and each said successivelayer consists of at least two tape-shaped wires. Advantageously, alayer of dielectric material separates each of the at least two distinctgroups of tape-shaped wire layers. Preferably, a layer of dielectricmaterial separates the core member from the layer of tape-shaped wiresclosest thereto. Advantageously, the dielectric material is selectedfrom the group consisting of polypropylene, polyethylene, andpolybutylene. In one embodiment of the present invention the at leasttwo distinct groups of tape-shaped wire layers carries approximatelyequal amounts of the current flowing through the cable. Alsoadvantageous is where the group of tape-shaped wire layers furthest fromthe core member provides shielding of the current flowing through theother layers and reduces magnetic fields or eddy currents in the cable.Preferably, the stabilizing metal used in the present invention isselected from the group consisting of silver, silver alloys, and nickeland nickel alloys, which may require a buffer layer.

Included in the present invention is an embodiment in which eachtape-shaped multifilamentary oxide superconducting wire has such astructure that is a number of filaments consisting essentially of anoxide superconductor contained in a stabilizing material of silver,silver alloys, nickel and nickel alloys. The oxide superconductor may beprepared from an oxide superconductor such as bismuth, strontium,calcium and copper oxide.

Advantageously, each of said plurality of layers contains at least 2tape-shaped silver contained wires per layer. Preferably, each of saidplurality of layers contains at least 4 tape-shaped wires per layer. Oneembodiment of the present invention includes an insulating layer betweenthe second and third layer of said plurality of layers. Where there aremore than 4 layers, advantageously, an insulating layer is presentbetween each second and third layer of said plurality of layers.

In the inventive conductor, the core member, which is generally called aformer, is adapted to hold the tape-shaped superconducting wires at abending strain of the prescribed range. This former has a length whichis required for the superconducting cable conductor, and is provided atthe center of the superconducting cable conductor. The former is in asubstantially cylindrical or spiral shape so that the tape wires arelaid thereon, and generally has a substantially constant diameter alongits overall length. The former can consist essentially of at least onematerial selected from the group consisting of metals such as stainlesssteel, copper, aluminum and the like and plastics, reinforced plasticsand ceramics.

According to the present invention, the former is preferably in the formof a tubular member having flexibility. It is also possible to employ apipe having a spiral groove (hereinafter referred to as a spiral tube)as a former having sufficient strength and flexibility. A bellows tubehaving a bellows may also be employed as a former. Further, the formercan also be prepared from a spirally wound material such as a spiralsteel strip. Each of these shapes is adapted to provide the former withsufficient flexibility. The flexible former provides the inventiveconductor with flexibility. The flexible conductor of the presentinvention can be taken up on a drum.

When practicing the present invention, it is possible to lay or windseveral tape-shaped multifilamentary superconducting wires on theformer. The tape wires may be laid in two or more layers while directinga surface thereof to the former. Each layer may be formed by anarbitrary number of the tape wires. When several tape wires are laid onthe former in parallel with each other so that the surface of the formeris filled up with the tape wires, additional tape wires are furtherwound thereon. When a sufficient number of tape wires are wound on thefirst layer of the tape wires as a second layer, a third layer of tapewires are then wound thereon. No insulating layer is provided betweeneach adjacent pair of layers.

In the present inventive method, each tape-shaped multifilamentary oxidesuperconducting wire is laid or wound on a former having a prescribeddiameter at a bending strain or a curvature of a prescribed range and apitch of a prescribed range. A relatively loose bending is applied tothe tape wire along its longitudinal direction. The tape wire which iswound on the former is bent at a bending strain of not more than 0.4%,preferably not more than 0.3%.

Superconductivity of the tape wire is not substantially reduced uponbending at a bending strain of such a range, as compared with that in alinear state.

The present invention it is preferable to adjust the pitch and thediameter of the former so that the bending strain of the superconductivewire is not more than 0.2%. Each tape-shaped multifilamentary oxidesuperconducting wire is preferably wound on the former with tension ofnot more than 2 kgf/mm.⁻² in a range of 0.5 to 2 kgf/mm⁻².

The core member (former) can be formed by either an electric insulatingmaterial or an electric conductor. The electric insulating material ispreferable in consideration of reduction in AC loss, while a metal whichis a conductor is preferable in consideration of strength. A metal pipehaving a spiral groove or a metal bellows tube may be used as the coremember for providing the conductor with flexibility while maintainingconstant strength. A metal core member can also be employed for safetyin the case of an accidental abnormal current. In this case, it ispossible to set optimum resistivity of the core member in considerationof AC loss of the conductor and the core member for the abnormalcurrent.

When a metal pipe, which optionally may have a spiral groove, or a metalbellows tube is employed as the core member, the conductor can furthercomprise a metal tape which is laid or wound on the core member, anddielectric tape which is laid on a the outside surface of the metaltape. The metal tape can form a smooth surface for covering any groovesof the core member so that the superconducting tapes do not buckle. Itis possible to cover any grooves while maintaining flexibility of thecore member by laying the metal tape.

According to the present invention, it is possible to employ tape-shapedmultifilamentary wires each having twisted filaments. The filamentsforming a superconducting multifilamentary tape are twisted at aprescribed pitch. Due to such twisting of the filaments, an inductioncurrent flowing between a stabilizing metal and the filaments is partedevery twisting pitch into small loops, and hence the value of thecurrent is limited. Thus, generation of Joule heat is suppressed in thestabilizing metal and AC loss is reduced as compared with asuperconducting wire having untwisted filaments.

The superconducting cable conductor according to the present inventionhas such flexibility that its superconductivity is substantially notdeteriorated also when the same is bent up to 50 times the diameter ofthe cable. This conductor can be wound on a drum, to be stored and/ortransported.

The present invention also makes it is possible to provide a long oxidesuperconducting cable conductor having flexibility as well as excellentsuperconductivity. In the present invention, an eddy current or acoupling current transferred between and flowing across thesuperconducting tapes is suppressed by the second or subsequent layer oftube-shaped superconductive wires which is provided according to oneembodiment of the present invention. The present invention provides apractical AC superconducting cable conductor.

Advantageously the superconductor material is a granulated ceramicinserted into a silver tube which is then drawn to about 1 to about 2mm. A number, depending on the desired capacity of the final cable, ofthese small drawn tubes are then inserted into a silver tube which isdrawn to the desired size for use. Optionally, such tube may first becut into sections and then added to the second silver tube beforedrawing. This thin, silver, flat tape-shaped material is from about 80to about 60 percent silver and about 20 to about 40 percent ceramic byweight, advantageously, about 65 percent silver and about 35 percentceramic.

The present invention also relates to a novel process or method whichproduces polymeric tapes suitable for use in a cryogenically operatedsuperconducting power cable and the tapes so produced. The processingincludes biaxially orienting either a polyethylene, polypropylene, orpolybutylene film which has a maximum dielectric constant of about 3.0and embossing said film with a random pattern. The combination of lowdielectric constant, biaxially oriented, embossed film yields apolymeric material which overcomes the problems of brittleness, crazing,and excessive shrinkage which renders polymeric materials produced byknown processes unusable in cryogenically operated power cable systems.In addition, the embossing of the film permits the relatively free flowof dielectric fluid within the cable.

The polyolefin sheet stock is biaxially oriented before use in the cableof the present invention. This involves stretching the sheet to a drawratio of between about 5 to 1 and about 10 to 1 in the length directionand also orienting the sheet across their width.

The sheet, and tapes obtained therefrom which results from processingpolyolefin stock to appropriate draw ratios has numerous qualities whichmake it superior for cable manufacture. To reduce the tape's tendency tofibrillate, to split over its entire length along a single tear, furtherprocessing is desirable. This processing involves a biaxial orientationin the direction across the sheet. This orients the sheet to a ratio ofup to about 50% in the cross-sheet direction, and produces tape which issufficiently biaxially oriented to satisfactorily limit the tendency tofibrillate.

Advantageously the superconducting cable of the present inventionincludes embossing the biaxially oriented dielectric material so as toform irregular and/or random channels therein. Preferably the dielectricmaterial is embossed with channels having a depth of from about 0.5 toabout 2 ml. In a preferred embodiment the embossing is performed by aroller at a temperature from about 80° C. to about 140° C. and thedielectric tape is embossed in a pattern which preferentially permitsimpregnant flow across the tape width. Most advantageously thedielectric tape is embossed in a pattern of irregular hills and valleysrunning across the tape.

The polyethylene, polypropylene and polybutylene tapes produced from theprocessing noted above are embossed with a particular pattern underspecific conditions to assure proper cable impregnation and heattransfer. The embossing pattern consists of random or irregularchannels, primarily directed in the cross machine direction. The tapesare cut from or otherwise obtained from the oriented sheet and may beused as single or multiple layer or laminate tapes.

At the same time the pattern, while it may permit some impregnant flowin both the machine and cross-tape direction, favors cross-tape flow andflow between butt gaps because such flow enhances impregnation fromlayer to layer and encourages heat transfer by convection. The cableitself is constructed of multiple layers of polyolefin tape, eitherpolyethylene, polybutylene or polypropylene. To facilitate cablebending, different widths of polyolefin tape may be used in the layers.The sizes may progress to larger widths with increased distance from theconductor of the cable.

The polyethylene, polypropylene, or polybutylene film of the presentinvention has a dielectric constant no greater than about 3.0, withabout 2.3 being the preferred maximum. The first processing stepconsists of biaxial orientation, or drawing, advantageously at a ratioof from about 5:1 to about 6:1 in the machine direction and up to about2:1 in the cross machine direction. Following orientation, the orientedtape is embossed at a temperature of from about 80° C. to about 140° C.,which produces on the tape a pattern consisting of irregular or randomchannels primarily directed in the cross machine direction.

Polymeric tapes which have not undergone the novel processing stepsdescribed above have several inherent problems which make them unusablein cryogenically operated superconducting power cable systems. Forexample, in a liquid nitrogen environment at 77° K., most polymerictapes become glass hard. This will lead to either tensile failure due tothermal contraction exceeding the inherent elongation or to simpledisintegration of the tape. Another problem is crazing in liquidnitrogen. Liquid nitrogen, with a boiling point of 77° K., is known tobe a powerful crazing agent for polymers. Crazing usually leads tostress cracking and ultimately fracture of the tape. The biaxialorientation process described above overcomes these problems ofbrittleness, excessive shrinkage, and crazing.

Many polymers exhibit two distinct modes of yielding. One type ofyielding involves an applied shear stress, although the yield phenomenonitself is influenced by the normal stress component acting on the yieldplane. The second type of yielding involves yielding under the influenceof the largest principal stress. This type of yielding is frequentlyreferred to as crazing, or normal stress yielding. Crazing can beinduced by stress or by combined stress and solvent action. It showsgenerally similar features in all polymers in which it has beenobserved. Crazing appears to the eye to be a fine, microscopic networkof cracks almost always advancing in a direction at right angles to themaximum principal stress. Crazing generally originates on the surface atpoints of local stress concentration. In a static type of test, itappears that for crazing to occur the stress or strain must reach somecritical value. However, crazing can occur at relatively low stresslevels under long-time loading.

It is known from extensive electron microscopic examination of crazedareas that molecular chain orientation has occurred in the crazedregions and that oriented fibrils extend across the craze surfaces.

To aid in the construction of the cable the otherwise highly transparentpolyolefin insulating tape advantageously is produced with coloringadded. This technique adds significantly to the ability to make auseable cable, because the operator must properly index each subsequentspiral layer of tape with the immediate previous layer. When taping withthe typical extremely clear and transparent polyethylene, polybutyleneor polypropylene tape, the operator is unable to distinguish the buttgaps of the immediate previous layer from other butt gaps as far aseight or ten tape layers beneath. The addition of selected color dyes inspecific quantities adds enough color to the tape to permit the operatorto easily distinguish the edges, the butt gaps, of the immediateprevious layer of tape from those of the earlier layers because thedarkness of the color increases significantly with each layer. Thiscoloring agent is selected so as to minimize any increase in dissipationfactor of the original material.

The width of the tapes may vary; narrow near the conductor and wider atthe outside. The direction of lay may also be reversed at a certainradial thickness, a factor which depends on the design of the tapingmachine.

The dielectric tapes may be wound in overlapping spiral layers so thateach butt gap between spirals of the same layer is offset from the buttgap of the layer below. This construction is facilitated by theproduction of the insulating tape containing color.

Polyolefin tapes such as polyethylene, polybutylene and polypropylene,when highly oriented as required for the present invention, aretransparent. This clarity becomes a disadvantage when the butt gaps ofmany layers show through to the surface of, the cable very clearly. Theoperator then has difficulty distinguishing the butt gap of theimmediate previous layer, from which each new butt gap must be offset,from other butt gaps deeper within the cable.

The tape of the present invention therefore has a color component addedto it so that the deeper a layer is within the cable, the darker itappears. Organic dyes may be used to produce this color because theseorganic compounds, unlike inorganic metal salts, have less detrimentaleffect on the loss tangent and permittivity of the tape.

Since a balance between the needed color and effects on the electricalcharacteristics must be struck, organic dyes are added in theproportions ranging between 100 to 1000 parts per million.

This results in a reduction in the light transmission of the tape to 10to 50 percent of the original transmission. When the tape is used on acable this reduces the visibility to one to four layers, whereas withoutcolor, butt gaps as deep as eight to ten layers within the insulationare, still visible.

Orientation is accomplished in the machine direction by stretching ortentering of the sheet to produce a thickness reduction ratio of between5 to 1 and 10 to 1.

The thickness reduction ratio is in fact a measurement of the linearsheet orientation and is an indication of the changing tensilecharacteristics of the polymer. The process is advantageously performedat temperatures of between about 80° C. and about 140° C.

The sheet is also processed to orient it in the cross-sheet direction toa reduction ratio of up to 50%. This is necessary because without suchprocessing polymers tend to fibrillate, that is, to separate intoindividual fibers across their width and cause the tape to splitlengthwise.

Polyolefin tapes resulting from the processing specified above, however,have a tensile modulus of at least 250,000 psi in the length (machine)direction, and meet all the criteria required for cable manufacture.

The tensile strength attained by the tapes through the processing is notonly an indication of the resistance to deterioration, but also anecessity for the use on cable taping machines. Tapes processed asdescribed above can therefore be used on conventional cable makingmachines with tensions great enough to construct a satisfactory tightlywound cable.

Before final construction into a cable, the polyolefin tape is embossedto furnish spacing between the tape layers which will facilitaterelatively free flow of impregnants within the cable to enhance heattransfer.

These goals are accomplished by a specific embossing technique. The tapeis embossed advantageously by rollers. A typical pattern of embossing isshown in FIG. 4 which is a top view of a small section of tape 60 withvalleys 61 in the pattern shown as dark lines.

The embossing pattern is characterized as irregular and preferentiallypermitting cross-tape flow of impregnant as opposed to flow along thelength of the tape. The pattern of irregular valleys running essentiallyacross the tape width as seen in FIG. 4 meets these criteria and, unlikea pattern of regular grooves or channels, it can not interlock adjacenttape layers. Non-uniform and irregular patterns therefore assure thatthe various tape layers can move small distances relative to each otherand yield the degree of flexibility required to manufacture and installthe cable.

The cross-flow favoring pattern provides heat transfer and impregnationcapabilities for the cable. Although it is well understood that polymersare not permeable, the mechanism available for impregnation and heattransfer in the present cable does not depend upon the permeability ofthe material itself.

The embossed pattern is such that it can increase the effective tapethickness, that is, the peak to peak thickness may be twice the distanceof the original tape thickness. The tape is then compressed duringwinding. Embossing is accomplished by rollers which cause a depressionin one surface of the tape and a protrusion in the other surface. Oncewound into a cable, these surface irregularities separate the tapelayers; but since the pattern favors across-the-tape flow, impregnantsneed only flow, at the most, one-half the width of the tape to or from abutt gap where it can then progress to the next space between the tapes.This results in a relatively short path from the outside of the cable tothe conductor.

Two typical patterns of embossing are: a coarse pattern with a typical0.1 mm mid-height width of the valleys and a typical 0.2 mm spacingbetween adjacent peaks; and a fine pattern with typical 0.025 mmmid-height valley widths and typical 0.05 mm spacing between peaks.

The availability of embossing patterns ranging from coarse to fineallows the cable designer to strike a compromise between heat transferand operating stress. The coarse pattern provides the best heat transferwith some reduction in operating voltage stress compared to the finepattern and vice versa.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A cable employing an oxide superconductor, comprising: a flexiblecore member; a plurality of tape shaped oxide superconducting wiresbeing laid on said core member with tension of not more than 2 kgf/mm²wherein each tape-shaped superconducting wire consisting essentially ofan oxide superconductor and a stabilizing metal covering the same, saidplurality of tape-shaped superconducting wires forming a plurality of atleast two distinct groups of tape-shaped wire layers each being formedby laying a plurality of said tape-shaped superconducting wires in aside-by-side manner, wherein a layer of dielectric material separateseach of the at least two distinct groups of tape-shape wire layers andthe dielectric material is biaxially oriented at a ratio from about 5:1to about 10:1 in the machine direction, said plurality of layers beingsuccessively stacked on said core member without an insulating layerbetween said plurality of layers and said core member, said core memberproviding said superconducting cable with flexibility, saidsuperconducting cable capable of maintaining a superconducting state atthe temperature of liquid nitrogen, said wires having substantiallyhomogeneous superconducting phases along the longitudinal direction ofsaid wire, the c-axes of said superconducting phases being orientedsubstantially in parallel with the direction of thickness of said wire,said superconducting wires being formed by grains aligned in parallelextending along the longitudinal direction of said wire, said grainsbeing stacked along the direction of thickness of said wire wherein saidcore member is a pipe having a mat shaped surface on its exterior whichforms a surface for the tape-shaped superconducting wires.
 2. Thesuperconducting cable of claim 1 having flexibility such that thesuperconductivity of said cable does not substantially deteriorate uponbending up to about 50 times the diameter of the cable.
 3. Thesuperconducting cable of claim 1, wherein said core member is selectedfrom the group consisting essentially of metals, plastics, reinforcedplastics, polymers, and composites.
 4. The superconducting cable ofclaim 1, wherein said wires are twisted within said tape-shapedstabilizing metal covering.
 5. The superconducting cable of claim 1,wherein said tape-shaped wires are laid at a lay angle of up to about 90degrees.
 6. The superconducting cable of claim 5, wherein saidtape-shaped wires are laid at a lay angle of from about 10 to about 60degrees.
 7. The superconducting cable of claim 6, wherein saidtape-shaped wires are laid at a lay angle of from about 20 to about 40degrees.
 8. The superconducting cable of claim 1, wherein the lay angleof each successive layer of tape-shaped wires alternate in lay directionor pitch.
 9. The superconducting cable of claim 8, wherein each saidsuccessive layer consists of at least two tape-shaped wires for aconstruction of four or more even layers.
 10. The superconducting cableof claim 1, wherein a layer of dielectric material separates the coremember from the layer of tape-shaped wires closest thereto.
 11. Thesuperconducting cable of claim 10, wherein the dielectric material isselected from the group consisting of polypropylene, polyethylene andpolybutylene.
 12. The superconducting cable of claim 1, wherein the atleast two distinct groups of tape-shaped wire layers carriesapproximately equal amounts of the current flowing through the cable.13. The superconducting cable of claim 12, wherein the group oftape-shaped wire layers furthest from the core member provides shieldingof the current flowing through the other layers and reduces magneticfields or eddy currents in the cable.
 14. The superconducting cable ofclaim 1, wherein the first of the two distinct groups of tape-shapedwire layers carries greater than 50 percent of the current flowingthrough the cable.
 15. The superconducting cable of claim 1, wherein thesecond of the two distinct groups of tape-shaped wire layers carriesgreater than 50 percent of the current flowing through the cable. 16.The superconducting cable of claim 1, wherein the stabilizing metal isselected from the group consisting of silver, silver alloys, nickel andnickel alloys.
 17. The superconducting cable of claim 1, wherein each ofsaid plurality of layers contains at least 2 tape-shaped wires perlayer.
 18. The superconducting cable of claim 1, wherein each of saidplurality of layers contains at least 4 tape-shaped wires per layer. 19.The superconducting cable of claim 18, including an insulating layerbetween the second and third layer of said plurality of layers.
 20. Thesuperconducting cable of claim 18, including an insulating layer betweeneach second and third layer of said plurality of layers.
 21. Thesuperconducting cable of claim 1, wherein the dielectric material has amaximum dielectric constant of about 3.0.
 22. The superconducting cableof claim 21, wherein the dielectric material has a maximum dielectricconstant of about 2.3.
 23. The superconducting cable of claim 1, whereinthe dielectric material is biaxially oriented at a ratio of from about5:1 to about 6:1 in the machine direction.
 24. The superconducting cableof claim 1, wherein the dielectric material is further biaxiallyoriented up to about 2:1 in the cross machine direction.
 25. Thesuperconducting cable of claim 24, wherein the dielectric tape isembossed in a pattern which preferentially permits impregnant flowacross the tape width.
 26. The superconducting cable of claim 1,including embossing the biaxially oriented dielectric material so as toform irregular and/or random channels therein.
 27. The superconductingcable of claim 26, wherein the dielectric material is embossed withchannels having a depth of from about 0.5 to about 2 mm.
 28. Thesuperconducting cable of claim 26, wherein the embossing is performed bya roller at a temperature from about 80° C. to about 140° C.
 29. Thesuperconducting cable of claim 26, wherein the dielectric material isembossed in a pattern of irregular hills and valleys running across thematerial.
 30. The superconducting cable of claim 26, wherein thedielectric tape is embossed in a pattern which increases the effectivetape thickness.
 31. The superconducting cable of claim 26, wherein thedielectric tape is embossed in a pattern with up to about 0.2 mm spacingbetween the adjacent peaks.
 32. The superconducting cable of claim 31,wherein the dielectric tape is embossed in a pattern with up to about0.05 mm spacing between peaks.
 33. The superconducting cable of claim 1,wherein the dielectric tape is produced from material which containsorganic color dye in a quantity within the range of 100 to 1000 partsper million.
 34. The superconducting cable of claim 1, wherein thedielectric tape has a tensile modulus of at least 250,000 psi.